Thyratron switch



April 21, 1959 R. CREVELING ETAL 2,333,535

THYRATRON SWITCH Filed March 15, 1956 2 She ets -Sheet 1 ll-Q ofagfe Deec ar Fig.

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I //v VEN rons Roberf. Crave/[0g Nolan A. Bourgeo/s, Jr.

Alto/nay April 1959 R. CREVELING' ETAL 2 ,883,535

KTHYRATRON SWITCH Filed March 15, 1956 2 Sheets-Sheet 2 I Tfiyratran R-F Source Anfarma Fig. 7

m/ VE/V TORS Haber) Cre val/0g Nolan A. Bourgeois, Jr.

A from ey United States Patent THYRATRON SWITCH Robert Creveling andNolan A. Bourgeois, Jr., Albuquerque, N. Mex., assignors, by mesneassignments, to the United States of America as represented by theUnitedStates-Atomic Energy Commission Application Marc 15, 1956, SerialNo". 571,845 2 Claims. c1. 250-47 This invention relates to thyratronsin general, and more particularly to a method and means of utilizing athyratron as an efiicient electrical switch. Thyratrons have been usedfor years in the control of large amounts of electrical power, as inwelding, lighting control, motor control and voltage regulation andcontrol of rectifiers. Recently, coincident with the general growth ofelectronics, the use of thyratrons in the control of much smalleramounts of power has greatly increased. The thyratron is a fast-acting,compact switch which has no moving parts and is perfectly satisfactoryin many electronic applications, particularly those which require thetube to carry a large current.

However, when the thyratron is required to carry only afraction of itsmaximum rated current, the very action by which conduction is maintainedwithin the tube acts against the efliciency of the switch. Duringlow-current conduction the voltage drop across the tube oscillates at anirregular rate, spending a certain amount of time at or slightly abovethe ionizing potential to produce a supply of ions, then falling to amuch lower value, near zero volts, until this supply is used up. Ingeneral, the time spent producing the ion supply is much less than thetime duration of the lower voltage. As a result, irregularly spacedvoltage spikes, generally referred to as noise, are observed between theplate and the cathode of the thyratron.

Since the ionization potential of xenon, a gas commonly used inthyratrons, is 12.08 volts, it is seen that these noise spikes caneasily prove annoying in electronic triggering circuits. Voltages ofthat order of magnitude are commonly used as control voltages both forthyratrons and for other electron tubes. In particular, where thyratronsare used in electronic circuits which perform critical timing operationsinvolving times of the order of microseconds, the noise generated by thethyratrons can well be sufficient to considerably reduce the accuracyand repeatability of the operations.

Therefore it is an object of this invention to provide a method ofutilizing a thyratron as an electrical switch which has a low voltagedrop and which introduces very little noise into the signal beingswitched.

Another object of the invention is to provide various means ofpracticing the above method according to the demands of the associatedcircuitry.

Briefly, the method practiced in this invention is to create asuperabundance of ions within the thyratron sufiicient to prolong thetime duration of the low tube drop between noise spikes. During thistime the signal current is conducted through the tube, depleting the ionsupply.

One embodiment of the invention practices the above method by providinga capacitor which is previously charged to a high potential, thendischarged through the thyratron to produce a superabundanc'e of ions tobe used during the later conduction of a signal current.

, Other embodiments utilize a pulse generator whose output terminals areconnected between two elements of the thyratron so that heavy currentpulses are passed between those elements, producing the desiredsuperabundance of ions.

Still another embodiment includes a radio-frequency generator arrangedso that an ionizing electromagnetic field is passed through thethyratron, producing sufiicient ions to prolong the noise-freeconduction of the tube.

If it is necessary to conduct a signal current for a longer period oftime than is afforded by the supply of ionsproduced by any of thedescribed embodiments, those embodiments using the pulse generator andthe radio-frequency generator are particularly adaptable to the emissionof a series of ionizing pulses so spaced in time as to successivelyreplenish the supply of ions.

A better understanding of the invention may be had, and other objects ofthe invention will become apparent, by reading the following moredetailed description in conjunction with the appended claims and theattached drawings in which:

Fig. 1 is the circuit diagram of a time-delay circuit incorporating theinvention as embodied in a capacitor-discharge device for ionizing thegas within a thyratron;

Fig. 2 shows the voltage waveform observable at point a of the circuitof Fig. 1 during operation of the circuit;

Fig. 3 shows the voltage waveform observable at point b of the circuitof Fig. 1;

Fig. 4 shows the output voltage pulse of the circuit of Fig. 1;

Fig. 5 is the circuit diagram of an embodiment of the inventionutilizing a pulse generator to provide ionizing pulses between the gridand the cathode of a thyratron; and

Fig. 6 is the circuit diagram of another embodiment, which includes apulse generator connected to supply ionizing pulses between the plateand the cathode of a thyratron.

Fig. 7 shows an arrangement of apparatus for the application of anelectromagnetic field to a thyratron.

Referring now to Fig. 1, which shows the invention incorporated in atime delay circuit claimed and more completelydescribed in a copendingapplication of Robert Creveling, Serial Number 571,844, filed March 15,19 56, thyratron 10 is in the non-conducting state, maintained that wayby the C-bias on control grid 19. Capacitors 12 and 13 are each chargedpositively at B+, the plate supply voltage. At time T tube 10 istriggered into the conducting state by a positive pulse applied betweenterminals 14. The plate of tube 10 drops to practically ground potentialdue to the voltage drop across resistor 24. Capacitor 13 then dischargesthrough current-limiting resistor 15 and tube 10 to ground in less thana microsecond. The surge of current, which is near the maximum tuberating, creates a large number of ions within the tube so that thesubsequent voltage drop from plate to cathode due to the lesser signalcurrent is low and free of oscillations. The side of capacitor 12 thatwas at a high potential is now only a few volts above ground.

Fig. 2 shows the potential of point a in Fig. 1 as it varies with time.It is seen to drop quickly from ground potential to a voltage equal invalue but having opposite polarity to B+ when tube 10 fires, then torise to near B+ following a sinusoidal variation to the extent of half acycle. The duration of this half-cycle of oscillation, which is thedesired time delay, is determined by the values of capacitor 12 and coil16. The circuit involved includes tube 10, the forward resistance ofdiode 17, coil 16, and capacitor 12. The relatively small currentflowing through tube 10 during the time delay uses the supply of ionscreated in tube 10 by the discharge of capacitor 13. Until this supplyis exhausted, the tube does not attempt to replenish it and no noise isadded to the waveform of Fig. 2.

It is particularly important that this waveform be noise free near itsmost positive point, which corresponds to the end of the time delay,because of the triggering action to follow. When the voltage at point 01reaches its peak and starts to decrease in a second half-cycle, currentin the oscillatory circuit is prevented from reversing by diode 17,since it can conduct efliciently in only one direction. Hence, theenergy stored in coil 16 is dissipated by decaying voltage oscillationsat a frequency determined by the coil inductance and its straycapacitance.

Fig. 3 shows this voltage as observed at point b of the circuit. Thesudden rise in voltage at the start of the oscillations triggers voltagedetector 11, causing an infiection in the waveform, and the output pulseshown in Fig. 4, observable at terminals 18, is produced at the desireddelay time after the signal pulse.

This time delay circuit is designed to produce a delay of the order oftens of microseconds between input and output pulses. It is seen thatthe occurrence of a noise pulse near the triggering time of voltagedetector 11 could easily cause early or late triggering, creating avariation in the time delay. In the prior art such noise-causedvariations were commonly between three and 250 millimicroseconds. Theuse of this invention as shown in Fig. 1 yields time delays havingvariations of less than three millimicroseconds.

In Fig. 1, thyratron 10 is shown to be the type having four elements,such as the 2D21, although the invention may instead include athree-element thyratron. Grid 19 is that usually referred to as gridnumber one or control grid, for which capacitor 20 and resistor 21 formR-C coupling from terminals 14.

Grid 22 is that usually called grid number two or shield grid and isnormally connected to ground or to the cathode. However, in thisinstance it is used to supplement the triggering of the thyratron. Sincethe triggering voltage acts directly on grid 19, ionization is startedtherewith, closely followed by further ionization due to the triggeringvoltage reaching grid 22 by way of resistor 23, which is of the order ofa thousand ohms.

Fig. shows the use of a pulse generator in producing one or moreionizing pulses. Here the input pulse is applied to the shield grid andthe pulse generator at the start of the time delay, firing the thyratronand triggering the pulse generator. The generated current pulse, whichmay be considerably larger than the input pulse, is applied to thecontrol grid and provides an overabundance of ions within the thyratron.The signal current from a source not shown then flows through thethyratron from plate to cathode, using the supply of ions.

If more ions are needed than can be supplied by one pulse from the pulsegenerator, a self-triggering generator may be used. This type ofgenerator would be turned on by the input pulse and would then continueto generate ionizing pulses at a repetition rate sufficient to preventthe exhaustion of ions while the signal current flows.

As shown in Fig. 6, the pulse generator may be connected between plateand cathode, in which case the generator can be free-running. Thegenerated pulses will have no effect on the ionization of the thyratronuntil it is fired by a positive pulse on one of the grids. In this casethe input pulse is applied to the shield grid, causing the tube toconduct at the desired firing time, and then successive pulses from thepulse generator keep the supply of ions replenished. The signal currentcan be caused to flow through the thyratron at any time after the tubefires, by any of various known means, not shown.

Since the construction of most thyratrons is such that the grids mayconduct substantial currents without damage, other connections betweenthe thyratron and the external circuitry are possible. If necessary,ionizing pulses may be applied between two grids, or between the plateand a grid. The pulses should be several hundred volts in amplitude toinsure ionization, since the ionization in this case depends upon theformation of a cold cathode at the tube element used as the negativeterminal of the pulsing circuit. Also, the requirements of a particularapplication may allow connections whereby a grid carries both the inputpulse and the ionizing pulse, permitting the use of a triode instead ofthe tetrode shown in Figs. 5 and 6.

Another embodiment of the invention, shown in Fig. 7, makes use of anelectromagnetic field to ionize the gas within-the thyratron. In thisembodiment a thyratron such as the 3C23, which has no shield around theplate, grid and cathode, is recommended, since ionization of the gasbetween those elements is more quickly efiected in the absence of such ashield. No separate triggering means is needed, since application of theelectromagnetic field to the tube will simultaneously fire the tube andprovide a superabundance of ions to be used in conduction of a signalcurrent.

Referring to Fig. 7, radio-frequency source 30 radiates anelectromagnetic field which is conducted by wave guide 31 to antenna 32.Thyratron 33 is supported before the antenna so that the radiatedelectromagnetic field passes through the thyratron, ionizing the gaswithin. The power of source 30 depends upon the degree of ionizationrequired and whether the electromagnetic field is applied in pulses asneeded to prevent noise or applied continuously.

A method of increasing the efiiciency of a thyratron switch has beendescribed, along with several specific embodiments which may be used inpracticing the method. It should be noted that besides being useful inthe micro second timing field, the invention will add considerably tothe efliciency of many other electronic circuits in which the presenceof thyratron noise is bothersome. Various changes in, and additions to,the method and devices described may be made by one skilled in the artwithout departing from the sphere and scope of the invention as definedin the claims below.

What is claimed is:

1. A thyratron switch having the operating characteristic of a prolongednoise-free time interval immediately following the start of switchconduction, comprising a thyratron having at least an anode, a cathodeand a control grid; a series circuit connected between the cathode andthe anode including a capacitor and a resistor (R) of such size relativeto a voltage (E) established across the capacitor as to limit acapacitor discharge current (I) at approximately the maximum ratedcurrent of the thyratron, the relationship applying; means forestablishing the voltage (E) across the capacitor; means for firing thethyratron; and means including the anode and the cathode forautomatically conducting a relatively small signal current compared tothe maximum current rating through the thyratron upon its firing and forlimiting the time duration of the signal current to a value short of thetime of occurrence of noise pulses in the thyratron, said thyratronhaving a characteristic rate of occurrence of noise pulses whichnormally would cause noise pulses to be introduced to the signalcurrent.

2. For use with a unidirectional series resonant timedelay circuitconnected between the anode and the cathode of a thyratron and soconstructed and arranged as to conduct a small fraction of the maximumrated current of the thyratron during the delay time, means for delayingthe occurrence of noise pulses within the thyratron until afterconduction of a half cycle of an oscillatory signal generated by saidtime-delay circuit, comprising a series circuit connected in parallelwith said time-delay circuit and including a capacitor and a resistor(R) of such size relative to a voltage (E) established across thecapacitor as to limit a capacitor discharge current (I) References Citedin the file of this patent at approximately the maximum rated current ofthe thyra- UNITED STATES PATENTS tron, the relationship E 1,138,652Graves May 11, 1915 I 5 2,210,523 Blumlein Aug. 6, 1940 2,470,303Grcenough May 17, 1949 applying, means f r es a l s g the voltage across2,538,267 Pierce et a1 Ian. 16, 1951 the capacitor, and means for firingthe thyratro i 2, 0 ,539 Bums July 22, 1952 thyratron having acharacteristic rate of occ rren f 45,50 winter July 21, 1953 noisepulses which normally would cause noise p l to 10 2,74 ,275 Maren et 1May 29, 195

be introduced to the signal current.

